To me, that depends on your pack, the gear you are carrying, and the type of pad you have.
Assuming you don't care what type of pad, it can depend on the pack.
Gossamer Gear packs (and some others) have a pack sleeve. Internal or external it really doesn't matter. (I also use a trimmed nightlite pad in GG packs to do this.) Too thick, and it rides the rest of the weight further out on your back. Due to leverage, not a good thing, generally. But, OK for <20#. For more weight, I wouldn't use these. Rather, a couple .5-10z stays (arrow shafts, or the like) work a bit better.
Some packs, like the Jam and ilk, like a rolled pad to create a tube like frame. This works OK, for light weights(<20#), also.
For heavier weights, 25#-30#, You can cut and tape a foam pad to shape, retaining the length (usually 1/2-3/4 or 36"-50") for additional structure. Cut exactly to the pack size, the pack and pad work quite well to create a structured pack to transfer weight to the belt/hips. This uses the interior space up pretty well, though. Soo you need about a larger 3000ci pack for a 5-7 day hike. The heavy stuff should go outside, in the pockets, since the pads will tend to push weight away from your back, but, they won't effect the side pouches. (water, fuel, tarps, etc.)
If you use any of the therm-a-rest self inflatable pads, these make OK frames, but you need some arrow shafts tucked into them for support. Alone, they work well for 15-18# but start failing therafter. The arrow shafts add about 8#-10# of support.
I cheat a bit, I use my fishing rod on one side and a tarp pole on the other, soo, they do double duty to some degree as part of the pack system. Soo, often times, you don't even need a pad for support.
You can also use a Z-Rest in the center to form a frame. Good for 10-15# loads, more with other tricks. I think SMD puts the pad pouch to one side for this, but I haven't tried his packs...but about the same idea.
I have toyed with splitting a Z-Rest, and, velcroing it back together when needed. Put each half on the inside of the pack, left and right. This works OK, too. Again, I was only carrying ~22 pounds, though.